1. Field of the Invention
This invention relates to process control systems. More specifically, the present invention relates to a process control system for controlling a plurality of devices using a control strategy that is implemented in a layered hierarchy of control modules.
2. Description of the Related Art
Present-day process control systems use instruments, control devices and communication systems to monitor and manipulate control elements, such as valves and switches, to maintain at selected target values one or more process variables, including temperature, pressure, flow and the like. The process variables are selected and controlled to achieve a desired process objective, such as attaining the safe and efficient operation of machines and equipment utilized in the process. Process control systems have widespread application in the automation of industrial processes such as the processes used in chemical, petroleum, and manufacturing industries, for example.
Control of the process is often implemented using microprocessor-based controllers, computers or workstations which monitor the process by sending and receiving commands and data to hardware devices to control either a particular aspect of the process or the entire process as a whole. The specific process control functions that are implemented by software programs in these microprocessors, computers or workstations may be individually designed, modified or changed through programming while requiring no modifications to the hardware. For example, an engineer might cause a program to be written to have the controller read a fluid level from a level sensor in a tank, compare the tank level with a predetermined desired level, and then open or close a feed valve based on whether the read level was lower or higher than the predetermined, desired level. The parameters are easily changed by displaying a selected view of the process and then by modifying the program using the selected view. The engineer typically would change parameters by displaying and modifying an engineer's view of the process.
In addition to executing control processes, software programs also monitor and display a view of the processes, providing feedback in the form of an operator's display or view regarding the status of particular processes. The monitoring software programs also signal an alarm when a problem occurs. Some programs display instructions or suggestions to an operator when a problem occurs. The operator who is responsible for the control process needs to view the process from his point of view. A display or console is typically provided as the interface between the microprocessor based controller or computer performing the process control function and the operator and also between the programmer or engineer and the microprocessor based controller or computer performing the process control function.
Systems that perform, monitor, control, and feed back functions in process control environments are typically implemented by software written in high-level computer programming languages such as Basic, Fortran or C and executed on a computer or controller. These high-level languages, although effective for process control programming, are not usually used or understood by process engineers, maintenance engineers, control engineers, operators and supervisors. Higher level graphical display languages have been developed for such personnel, such as continuous function block and ladder logic. Thus each of the engineers, maintenance personnel, operators, lab personnel and the like, require a graphical view of the elements of the process control system that enables them to view the system in terms relevant to their responsibilities.
For example, a process control program might be written in Fortran and require two inputs, calculate the average of the inputs and produce an output value equal to the average of the two inputs. This program could be termed the AVERAGE function and may be invoked and referenced through a graphical display for the control engineers. A typical graphical display may consist of a rectangular block having two inputs, one output, and a label designating the block as AVERAGE. A different program may be used to create a graphical representation of this same function for an operator to view the average value. Before the system is delivered to the customer, these software programs are placed into a library of predefined user selectable features. The programs are identified by function blocks. A user may then invoke a function and select the predefined graphical representations to create different views for the operator, engineer, etc. by selecting one of a plurality of function blocks from the library for use in defining a process control solution rather than having to develop a completely new program in Fortran, for example.
A group of standardized functions, each designated by an associated function block, may be stored in a control library. A designer equipped with such a library can design process control solutions by interconnecting, on a computer display screen, various functions or elements selected with the function blocks to perform particular tasks. The microprocessor or computer associates each of the functions or elements defined by the function blocks with predefined templates stored in the library and relates each of the program functions or elements to each other according to the interconnections desired by the designer. Ideally, a designer could design an entire process control program using graphical views of predefined functions without ever writing one line of code in Fortran or other high-level programming language.
One problem associated with the use of graphical views for process control programming is that existing systems allow only the equipment manufacturer, not a user of this equipment, to create his own control functions, along with associated graphical views, or modify the predefined functions within the provided library.
New process control functions are designed primarily by companies who sell design systems and not by the end users who may have a particular need for a function that is not a part of the standard set of functions supplied by the company. The standardized functions are contained within a control library furnished with the system to the end user. The end user must either utilize existing functions supplied with the design environment or rely on the company supplying the design environment to develop any desired particular customized function for them. If the designer is asked to modify the parameters of the engineer's view, then all other views using those parameters have to be rewritten and modified accordingly because the function program and view programs are often developed independently and are not part of an integrated development environment. Clearly, such procedure is very cumbersome, expensive, and time-consuming.
Another problem with existing process control systems is a usage of centralized control, typically employing a central controller in a network, executing a program code that is customized for specialized, user-defined control tasks. As a result, the process control systems are typically constrained to a particular size and difficult to adapt over time to arising needs. Similarly, conventional process control systems are inflexible in configuration, often requiring a complete software revision for the entire system when new devices are incorporated. Furthermore, the conventional process control systems tend to be expensive and usually perform on the functions initially identified by a user or a system designer that are only altered or reprogrammed to perform new functions by an expert who is familiar with the entire control system configuration and programming.
A further problem with existing process control systems is that the physical implementation of different systems is highly variable, including control devices and field devices that have a wide range of "intelligence". For example, some field devices, such as valves, motors and regulators, may have no computational or control capability. Other field devices may have a high level of control autonomy. Still other devices may have some computational strength, but not a sufficient amount to accomplish a desired control task.
What is needed is a uniform or universal design environment that can easily be used, not only by a designer or manufacturer but also a user, to customize a control process to the physical constraints of the process, utilizing control capabilities various controllers and devices, supplementing these control capabilities when desired and distributing control functionality flexibly throughout the process control system to meet specific needs for developing process control functions. What is further needed is a personal computer-based process control system that is easily implemented within substantially any size process and which is updated by users, without the aid of the control system designer, to perform new and different control functions by distributing these control functions throughout the control system including all central, intermediate and peripheral levels.